The invention relates to an apparatus and a method for heating a roll used in the manufacture of paper, such as a calender roll, inductively by means of eddy currents, in which apparatus pole shoe devices provided with electromagnetic coils are used.
The invention additionally relates to a roll heated electromagnetically by means of eddy currents, such as a calender roll so heated, as well as to the use of such a roll.
It is known in the prior art to heat calender rolls and drying cylinders in the drying section of a paper machine, by means of pressurized fluid or steam. Since temperatures higher than 100.degree. C. are desired in the heating, sealing problems are encountered in the use of steam. Moreover, conventional heated rolls are expensive to manufacture, because they must be manufactured as pressure vessels. The efficiency of heating rolls by means of a pressure medium is also poor, partially because large masses must be heated therein. Moreover, the adjustment of the temperature profile in the axial direction of the rolls causes difficulties.
In view of achieving certain particular objects, FI patent application No. 812697 discloses an electromagnetically heatable roll, in particular a calender roll, which is provided with a mantle of a ferromagnetic material and with a magnet inductively generating eddy currents in the same. In the arrangement disclosed in this patent application, several magnets have been fitted in blocks placed side by side in the axial direction, leaving at least the working zone within the outer circumference free. The set value corresponding to the change in the magnetic flux in the mantle of the roll can be adjusted separately within each block or groups of blocks. At least one temperature measurement-value detector is used, which indicates the measurement value corresponding to the actual value of the temperature of the outer surface of the roll mantle at different locations axially spaced from each other. A system of control circuits has been formed which changes the set values on the basis of the measurement values and of the temperature profile given in advance for the outer surface of the roll mantle.
However, it is a drawback of the heating arrangement of FI patent application No. 812697 that it is necessary therein to heat a roll having a relatively thick mantle. Also, the interior of the supporting roll mantle is also occupied by the electric heaters or equivalent, so that it is impossible to provide the rolls with the variable-crown equipment, which is itself known and which has been observed to be advantageous in practice and indispensable, e.g., in connection with calender rolls. In the above Finnish patent application, the most important aspect has been considered to be a closed control system, in which the temperature of the roll is detected at different points in the axial direction of the roll and, on the basis of these measurements, the adjustment of the temperature profile is carried out. However, since it is necessary in this regard, to use a roll having a relatively thick wall and to heat relatively large masses, a detrimental delay is caused in the control system.